Mono- and/or bis (mono- and/or di- and/or trichloromethyl)benzenes are important intermediates in the preparation of pesticides, colorants, herbicides and additives for plastics (UV stabilizers).
The usual technique for obtaining these chlorinated intermediates consists in carrying out the chemically or photochemically initiated chlorination of the methylbenzenes.
This reaction is generally carried out industrially with chlorine gas. As this reaction is that of a gas with a liquid, the overall kinetics of the reaction are controlled both by the chemical kinetics and the physical kinetics, that is to say the diffusion of the chlorine gas. The chemical kinetics depend essentially on the nature of the substituent or substituents on the benzene as well as on the initiation method.
The mechanism of the reaction is that of a radical chain reaction, the yield decreasing sharply between the various stages of the chlorination, that is to say from the starting --CH.sub.3 to the monochlorinated compound --CH.sub.2 Cl, then to the dichlorinated compound --CHCl.sub.2 and finally to the trichlorinated compound --CCl.sub.3.
Consequently, towards the and of the reaction, when the content of hydrogen atoms on the --CH.sub.3 capable of being substituted is relatively low, side reactions, such as chlorination on the ring, can take place.
In particular, these side reactions will increase in importance as the amount of available chlorine increases and when a sufficient amount of the radical species is no longer present.
In order to mitigate these disadvantages, provision is made, in Patent Application FR 2,156,911, relating to the preparation of trichloromethylbenzene, to dilute the excess chlorine with an inert gas, such as nitrogen, in order to decrease the undesirable side reactions.
In addition, long residence times (contact times) are used in order to achieve complete chlorination of the underchlorinated intermediates
However, the aforesaid way of operating exhibits the disadvantage that an excessively large amount of chlorine necessarily passes into the effluents, with the inert gas, which necessitates continuous treatment operations on the said effluents: if desired, to recover the gaseous hydrochloric acid produced during the reaction; or destruction operations; or, as in the abovementioned application, the use of a second chlorination plant, in as much as in FR 2,156,911, the photochemical chlorination of toluene is carried out in a cascade of reactors divided into two sections.
In a first section composed of 5 reactors, 2.3 to 2.9 mol of chlorine per mole of toluene are injected, i.e. 86 to 96% of the amount of chlorine theoretically necessary to prepare trichloromethylbenzene.
Subsequently, the mixture obtained is chlorinated in the second section, composed of 4 reactors, with an excess of chlorine diluted with an inert gas.
The effluents from the final reactors pass into the first reactor of the first section. The chlorine content in the exiting HCl is less than 1%.
In the second section, the final reactor has a volume which represents 2 to 3 times the volume of all the preceding reactors of the two sections.
However, this system does not make it possible to eliminate the by-products sufficiently, since the final product comprises approximately 5% of ring-chlorinated by-products.
Another known system is described in U.S. Pat. No. 4,056,455 of a continuous process for the photochlorination of toluene which consists in carrying out the reaction in a cascade of 10 reactors.
The toluene is introduced into the first reactor, the reactors 2 to 9 being fed with predetermined amounts of chlorine gas.
The effluents from the reactors 5 to 10, which are rich in chlorine, return to the reactors 2 and 3.
Likewise, the effluents from the reactors 2 to 4 are introduced into the reactor 1 comprising a high concentration of toluene, so that a large part of the chlorine present in the HCl in converted.
However, with this process, it is found that the gaseous hydrochloric acid still comprises a significant amount by weight of chlorine, which is still at most equal to 2% .
This amount of chlorine necessarily results in the chlorination on the ring of the methylbenzenes entrained when scrubbing out hydrochloric acid with water. This results in a poor quality of the hydrochloric acid solutions as it is difficult to remove the methylchlorobenzenes (heavy products) by stripping and virtually impossible to recycle the phase in which they are present.
Furthermore, it is found, in FIG. 1 of the patent in which the plant is shown diagrammatically, that the 10 reactors are represented in the same way. Consequently, there is nothing which could allow it to be supposed that the said reactors would be different in size.
Furthermore, it is generally accepted that oxygen has an effect on radical reactions, in particular on chlorination reactions of aromatic or aliphatic organic compounds, it being possible for this effect to be favourable or unfavourable to the progression of the reaction.
Thus, Serguchev Jv. A. et al. (Zhurnal org. khim, (1983) XIX, Vol. 5, pages 1020-1023) studied the action of oxygen on the rate of radical chlorination of the side chain of toluene.
They showed that, by carrying out the reaction with carefully deoxygenated toluene at temperatures of between 90.degree. C. and 130.degree. C. and by using chlorine with an O.sub.2 content in the region of 0.02% by volume, the chlorination of toluene to trichloromethylbenzene was carried out with a yield of 95% after 26 hours in the absence of chemical initiators and of light radiation.